1. Field of the Invention
This invention relates generally to fuel injection systems for internal combustion engines, and in particular to single point throttle body fuel injection systems designed for retrofitting vintage carburetion fuel delivery systems.
2. Background Art
A carburetion fuel delivery system uses a carburetor to supply and meter the mixture of fuel and air in relation to the speed and load of the engine. FIG. 1 illustrates a typical carburetor (10). Carburetor (10) includes one or more barrels (12). A butterfly-type throttle valve (18) is located near the bottom of the barrel (12), the opening and closing of which is controlled through a throttle linkage (not illustrated). Each barrel (12) includes a primary venturi (14) and an annular boost venturi (16), although additional venturis may be used to permit more precise metering of fuel and air under different conditions. Liquid fuel (20) is contained in a float bowl (22) and is in fluid communication with one or more orifices (21) located at the throat within the annular venturi (16). A jet (24) having a selectively sized port formed therethrough is disposed within the float bowl (22) at the entrance to the fluid passage (25) between the float bowl and the venturi (16). As air flows through the barrel (12) during operation of the engine (depicted using single-headed arrows), a low pressure develops at the throats of the venturis (14, 16) according to Bernoulli's law. The difference in pressure at the fuel across fluid passage (25) causes fuel to flow into the air stream (depicted using double-headed arrows). Orifices (21) atomize the liquid fuel, and because of the low pressure created by venturis (14, 16), the fuel is nearly instantaneously vaporized. The size of jet (24) determines the air/fuel ratio.
Variations in atmospheric temperature and pressure, engine temperature, load and speed make perfect carburetion nearly impossible to obtain under all driving conditions. A cold engine, an engine at idle, and an engine at wide-open throttle all require a rich fuel-air mixture, while a warm engine at cruise requires a lean fuel-air mixture. The airflow also varies greatly; the airflow through the carburetor at wide-open throttle may be 100 times greater than the airflow at idle. Complicating matters is the fact that gasoline has components with widely varying boiling points, which may result in less than fully vaporized fuel entering the engine cylinders under certain conditions, particularly when the intake manifold is cold.
In contrast, fuel injection systems meter fuel much more precisely than carburetors, thereby allowing optimal fuel-air mixture to be more consistently delivered across the full spectrum of driving conditions. Fuel injection provides increased horsepower, higher torque, improved fuel economy, quicker cold starting, and other benefits. As a result, fuel injection systems have largely replaced carburetion fuel delivery systems in automobiles manufactured after 1985.
Fuel injection systems use one or more fuel injectors, which are electromechanical devices that meter and atomize fuel. In each injector, application of an electrical current to a coil lifts a spring-loaded needle within a pintle valve off its seat, thereby allowing fuel under pressure to be sprayed through an injector nozzle to form a cone pattern of atomized fuel.
Fuel injection systems may be classified as single point, multi-point, or direct injection. As illustrated in FIG. 2, single point injection, also known as throttle body injection, uses one or more fuel injectors (64) located generally in a single location—the throttle body (62). Fuel is sprayed into throttle body (62) for delivery to the cylinders via the intake manifold (not illustrated). Fuel injectors (64) may be of the continuous injection variety, for which fuel is sprayed continuously and fuel delivery is controlled by adjusting fuel pressure, or of the intermittent injection variety, for which the injectors are rapidly cycled on and off and fuel delivery is controlled by the duration of the “on” pulse within a cycle. The latter variety is preferable for electronic control.
Although mechanical and hydraulic control systems are also known in the art, electronic control is the most common manner for governing the rate of fuel injection. A microprocessor- or microcontroller-based computer system is included within an engine control unit (ECU). The computer controls various engine and automotive systems as preprogrammed functions of numerous signals received from various sensors.
For control of fuel injection, the computer generates periodic pulse signals for each of the injectors, with “on” pulses for firing the fuel injectors. One or more driver circuits, located within the ECU, amplify and condition the pulse signals to be suitable for use with the fuel injectors. The cycle wavelength is a function of engine speed, and the pulse widths of the “on” pulses are a function of engine load. Engine speed is typically determined by a distributor output, a tachometer output, or a crankshaft sensor. Engine load is typically determined with either a mass airflow sensor or a manifold absolute pressure (MAP) sensor.
Based on the engine speed and load input signals, the computer generates the fuel injector pulse signals. The fuel injector pulse signals are initially based on target air-fuel ratio values, which are compensated for the volumetric efficiency of the engine at its operating speed and load. Target air-fuel ratios and volumetric efficiency coefficients may be stored in one or more look-up tables in volatile or non-volatile computer memory and are accessed using engine load and speed as input indices. The use of look-up tables allows for rapid response by the ECU to various vehicle operating conditions without the need for extensive time-consuming calculations. Controlling the fuel injection directly from the look-up tables is referred to as open-loop control.
However, when the ECU operates in a closed-loop control mode, the actual fuel injector pulse signals may vary from those derived directly from the look-up tables based on actual engine operating conditions. In closed-loop control, the amount of oxygen present in the exhaust gas is measured, which provides an indication of whether the engine is running too rich, too lean, or stoichiometrically. The fuel rate supplied to the engine is corrected by the ECU based on the input from an oxygen sensor so that the actual air-fuel ratio supplied to the engine equals the stored target air-fuel ratio under all conditions. In some ECU systems, one or more look-up tables may be updated based on the corrections derived during closed-loop control for better open-loop and closed-loop control. Closed-loop control is not used under some conditions, such as when the exhaust gas temperature is too cold for the oxygen sensor to provide reliable data.
There are a number of enthusiasts who operate vintage automobiles, often muscle cars, hotrods, and the like, who would benefit from upgrading the original carburetion fuel delivery systems with fuel injection systems. There is a desire, however, to maintain the traditional clean look, feel, and simplicity of a carburetor mounted atop the intake manifold. Throttle body fuel injection systems are ideal for such applications. Accordingly, a niche market has evolved for kits to adapt existing carburetors with injection capability or to replace existing carburetors with bolt-in-place throttle body fuel injection systems. Although such retrofit products exist, which provide many benefits of fuel injection, there is room for improvement in the way that fuel and air are delivered and mixed within the throttle body assembly.
For example, FIG. 2 is a perspective view of a throttle body fuel injection system (60) of prior art for replacing a carburetor, such as that disclosed by U.S. Pat. No. 7,735,475 issued to Farrell et al. on Jun. 15, 2010. A section of the throttle body (62) is broken out to reveal the structure of one of the air intake barrels or bores (72), a throttle valve (78), and the idle air control (IAC) circuit (80). The fuel injectors (64) are positioned so as to inject the fuel just above the throttle valve blades (78). The idle air circuit intake (82) is located at the top of the throttle body (62), and the outlet (84) is located at the bottom of the throttle body (62). An idle air controller motor (86) is connected to an IAC valve assembly (88) so as to allow air flow through the IAC circuit (80).
The Farrell et al. device positions the fuel injectors (64) just above the throttle blades (78) “to direct fuel to cover the upper surface of the throttle blade to improve fuel atomization.” U.S. Pat. No. 7,735,475, col. 3 ll. 58-59. Other designs, such as those disclosed by U.S. Pat. No. 5,809,972 issued to Grant on Sep. 22, 1998 or U.S. Pat. No. 4,348,338 issued to Martinez et al. on Sep. 7, 1982, utilize venturis akin to carburetor annular boost venturis (16) of FIG. 1 to create low pressure zones to improve atomization and vaporization of injected fuel. However, these designs may not provide optimal atomization and mixture delivery to each engine cylinder. Indeed, the use of venturis with concomitant low pressure zones in fuel injection systems has disadvantages, including imprecise fuel delivery due to the propensity to draw fuel out of the fuel passages downstream of the injectors during “off” periods in the fuel injection cycle and a greater risk for the accumulation of icing within the throttle body under certain conditions.
As another example, the Farrell et al. IAC circuit (80) is completely separate from the intake barrels (72). As a result, idle air flowing through the IAC circuit (80) is not mixed with fuel. For this reason, the mixture tends to be too lean during idle conditions, causing rough unstable idle. Analogously, in ECU systems of prior art, fuel injection and IAC algorithms are also independent of one another. IAC motor position is controlled primarily as a function of engine speed, and sometimes, coolant temperature. Additional inputs, such as manifold absolute pressure or throttle position, may also be considered to ensure that the engine is actually in an idle condition prior to actuating the IAC motor. Fuel injector pulsing is controlled primarily as a function of engine speed, engine load, exhaust oxygen levels, and sometimes manifold air temperature (for air density compensation), coolant temperature (i.e., for simulating carburetor choke function) or throttle position (i.e., for simulating carburetor accelerator pump circuit operation). Fuel injector pulsing is not a function of IAC motor position. As the IAC opens when the engine begins to idle, the fuel delivered to the engine, initially based on the open-loop look-up tables, becomes too lean. The ECU compensates for the lean idle condition during closed-loop control by measuring post-combustion oxygen levels, but any corrective feedback necessarily lags engine operation under undesirably lean conditions.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a fuel injection system for internal combustion engines that provides superior performance with optimal fuel distribution and idle control circuitry.
Another object of the invention is to provide an electronic fuel injection control system that provides superior performance during idle conditions.
Another object of the invention is to provide a fuel injection system for retrofitting carbureted engines that installs easily with minimal external connections.